1. Field of the Invention
The present invention relates to the art of electrochemical cells, and more particularly, to an improved electrochemical cell having a casing formed of dissimilar metals. More specifically, the present invention is of an electrochemical cell and manufacturing process thereof comprising an electrochemical cell enclosure or casing composed of dissimilar titanium alloys.
2. Prior Art
The recent rapid developments in small-sized electronic devices having various shape and size requirements require comparably small-sized electrochemical cells of different designs that can be easily manufactured and used in these electronic devices. Preferably, the electrochemical cell has a high energy density and is of a robust construction. Such electrochemical cells are commonly used to power automated implantable medical devices (AIMD) such as pacemakers, neurostimulators, defibrillators and the like.
One commonly used cell configuration is a secondary or rechargeable electrochemical cell. These secondary electrochemical cells are designed to reside within the medical device and remain implanted within the body over relatively long periods of time of up to 5 to 10 years, or more. As such, these secondary electrochemical cells are required to be recharged from time to time to replenish electrical energy in the cell.
Secondary electrochemical cells, such as those used to power automated implantable medical devices (AIMD), are commonly recharged through an inductive means whereby energy is wirelessly transferred from an external charging device through the body of the patient to the cell residing within the AIMD. Electro-magnetic (EM) induction in which EM fields are transmitted from an external charger to the cell within the AIMD is a common means through which the electrochemical cell is recharged. Thus, when the electrochemical cell requires recharging, the patient can activate the external charger to transcutaneously (i.e., through the patient's body) recharge the cell.
During the recharging process, a portion of the external charging unit comprising a plurality of charging coils is generally placed near the AIMD outside the patient's body. Due to this close proximity, the magnetic field produced by the charge coil(s) may induce eddy current heating of the cell enclosure or casing. Eddy current heating of the cell enclosure generally occurs when eddy currents, emanating from the charging coil, interact with the conductive material of the enclosure or casing. This interaction generates heat there within.
Eddy current heating results when a conductive material experiences changes in a magnetic field. In the case of recharging an electrochemical cell within an implanted medical device, eddy current heating occurs as the varying magnetic fields emanating from the coils of the external charging unit move past the stationary cell enclosure or casing. Eddy current heating is proportional to the strength of the magnetic field and the thickness of the conductive material from which the casing is manufactured. In addition, eddy current heating is inversely proportional to electrical resistivity and density of the casing material. Therefore, eddy current heating can be reduced by lowering the intensity of the magnetic field and the use of a casing material of increased electrical resistivity and reduced thickness.
As the AIMD is recharged, the phenomena of eddy current heating may result in excessive heating of the cell casing. This, therefore, could adversely affect the function of the electrochemical cell and/or the AIMD within which it resides.
Currently, device recharging rates and recharge time intervals must be limited to minimize the possibility of excessive heating. This results in reduced battery charge capacities, which increases the charging time interval. In addition, the number of recharging events may need to be increased to compensate for the reduced charge capacity. Therefore, the patient is required to recharge the electrochemical cell more frequently and for longer periods of time, thus equating to an overall longer recharging time.
Therefore, what is desired is an electrochemical cell enclosure or casing that minimizes eddy current heating and allows for increased charge rates and reduced charging times. In an embodiment of the present invention, reduction of eddy current heating is accomplished through the use of an enclosure or casing composed of a material comprising a relatively high electrical resistivity. Examples of such materials include Grade 5 titanium and Grade 23 titanium, which comprise various amounts of vanadium and aluminum. Specifically, these grades of titanium comprise about four percent vanadium and about six percent aluminum. As such, these materials exhibit relatively high electrical resistivities, which minimize eddy current heating.
However, these grades of titanium are generally known to be more refractive as compared to other materials, particularly other titanium alloys and, consequently, to exhibit increased brittleness and hardness. As a result, forming an enclosure of Grade 5 titanium or Grade 23 titanium is difficult. For example, forming processes used during the manufacture of a cell enclosure or casing such as drawing, forming, rolling, stamping and punching are limited due to the relatively increased brittleness of Grade 5 titanium and Grade 23 titanium.
Furthermore, the ability to withstand case deformation caused by normal swelling of the electrochemical cell over time is also limited. Such swelling and repeated stress cycling due to repeated charge-discharge cycles may crack the enclosure or casing, which may result in a breach of the cell's hermeticity. A loss of hermeticity could allow for leakage of electrolyte from the cell, which could damage the AIMD.
Therefore, what is needed is an electrochemical cell enclosure that is both mechanically robust and resistive to eddy current heating. The present invention addresses the shortcomings of the prior art by providing an electrochemical cell comprising an enclosure or casing that is both resistive to eddy current heating, mechanically robust, and easily manufacturable.